.content {
    width: 602px;
    height: 402px;
    margin: 100px auto 0;
    position: relative;
    border: 1px solid black;
    overflow: hidden;
    border-radius: 10px;
}
#oUl1 {
    position: absolute;
}
#oUl1 li {
    float: left;
    width: 600px;
    height: 400px;
    
}
#oUl1 li img {
    width: 600px;
    height: 400px;
    border-radius: 10px;
}
#oUl2 {
    position: absolute;
    bottom: 20px;
    left: 50%;
    transform:translateX(-50%);
}
#oUl2 li {
    display: inline-block;
    margin-left: 10px;
    width: 20px;
    height: 20px;
    border-radius: 50%;
    background-color: #ccc;
}
#oUl2 li:first-child {
    background-color: red;
}
.btn {
    position: absolute;
    top: 50%;

    visibility: hidden;
    transform: translateY(-50%);
    width: 80px;
    height: 0;
    transition: height 1s;
    overflow: hidden;
    text-align: center;
    line-height: 50px;
    font-size: 28px;
    background-color: rgba(255, 0, 0, 0.438);
    color: #fff;
    font-weight: 700;

}
.pre {
    left: 0;
}
.after{
    right: 0;
}
.content:hover .btn {
    visibility: visible;
    height: 50px;
}